Such absolute pressure sensors have not become known until now. The literature describes only those pressure sensors which are manufactured by semiconductor processes.
For instance, in the journal "J. Phys. E: Sci. Instrum.", Vol. 20 (1987), pages 1469 to 1471, a pressure sensor is described in which a polycrystalline silicon diaphragm is provided on a single-crystal silicon substrate, so that the diaphragm is not electrically insulated from the substrate. The diaphragm is first deposited on an SiO.sub.2 sacrificial layer, which is then removed underneath the diaphragm.
Thus, a space-charge region is formed at the boundary between polycrystalline silicon diaphragm and silicon substrate, so that the capacitance of this pressure sensor is strain- and temperature-dependent. Furthermore, the capacitance can be measured only by high-frequency sensing, not by low-frequency sensing.
In the journal "VDI Berichte", No. 939, 1992, pages 185 to 190, a high-pressure sensor is described in which a single-crystal silicon substrate supports a polycrystalline silicon diaphragm insulated from the substrate by means of a silicon-nitride film.
A similar pressure sensor with a polysilicon diaphragm insulated from the silicon substrate and a method of manufacturing such a sensor are described in DE-A-40 04 179.
As is stated in the journal "Sensors and Actuators", Vol. 28 (1991), pages 133 to 146, polysilicon films deposited on silicon exhibit built-in compressive strain. This results in a hysteresis of the pressure-capacitance characteristic and deteriorates the response of the pressure sensor to temperature changes.
In the case of a polysilicon diaphragm, the compressive strain in the as-deposited film can only be prevented by modifying the manufacturing process so that a defined tensile strain is produced in the diaphragm.
In the case of these prior art pressure sensors, and also in the sensors described in the journal "Sensors and Actuators", Vol. A21-A23 (1990), pages 1053 to 1059, the cavity is formed by first depositing an SiO.sub.2 sacrificial layer, then depositing the polysilicon diaphragm, and subsequently removing the sacrificial layer by etching in hydrofluoric acid.
This, however, has further disadvantages. After the etching in hydrofluoric acid, which is followed by rinsing in deionized water and drying, the thin polysilicon diaphragms generally stick to the silicon-substrate surface. This can only be prevented by taking complicated and costly countermeasures, which are explained in detail in the prior art mentioned above.
In addition, electrostatic fields, which are present at the surfaces of the silicon substrate and the polysilicon diaphragms a result of the etching and thereafter, lead to undesired deflection of the diaphragm. To eliminate this deflection, a bias voltage is necessary.
DE-A-37 23 561 describes a further semiconductor process for manufacturing a capacitive pressure sensor in which the sacrificial layer defining the cavity to be formed is etched away through openings in a first insulating layer which lie within the diaphragm area. Then, however, the material of the top electrode can penetrate into the cavity. This degrades the electrical properties of the pressure sensor.
To prevent this, a second insulating layer is provided which has a plurality of openings displaced in relation to the openings of the first insulating layer and prevents material of the top electrode from penetrating into the cavity. However, this second insulating layer complicates the manufacture considerably.
By contrast with the prior art, the problem underlying the invention is to provide resistive and capacitive absolute pressure sensors and processes for manufacturing same by surface-micromachining and thin-film techniques, with
the electrodes of the at least one capacitor in a capacitive thin-film absolute pressure sensor having a high insulation resistance relative to each other, PA1 the respective diaphragms in resistive and capacitive thin-film absolute pressure sensors exhibiting only little tensile strain in the finished condition, PA1 no sublimation step being necessary to prevent the diaphragm from sticking to the substrate, PA1 the diaphragm providing a measurement signal over a wide pressure range even if it rests against the substrate in the event of an overload, PA1 the measurement signal being virtually independent of temperature, and PA1 only few chemical-vapor-deposition and photolithographic steps being necessary. PA1 the base element supporting, on the cavity side, a substrate electrode having first interconnection tracks or corner pads extending therefrom, PA1 the diaphragm made of the material of a first insulating layer which firmly adheres, at least in part, to the base element at the edge of the cavity, PA1 the diaphragm supporting, on the side remote from the cavity, a top electrode and a second insulating layer which completely covers the top electrode and the diaphragm and hermetically seals the cavity, and PA1 the top electrode having second interconnection tracks extending therefrom onto the first insulating layer outside the diaphragm. PA1 the diaphragm made of the material of a first insulating layer which firmly adheres to the base element around the edge of the cavity, PA1 the diaphragm supporting, on the side remote from the cavity, a piezoresistive half bridge or a piezoresistive full bridge and a second insulating layer which completely covers said bridge and the diaphragm, and PA1 the piezoresistive half or full bridge having leads extending therefrom onto the base element outside the diaphragm. PA1 a) depositing a first metal layer over the entire surface of a glass substrate serving as the base element, said first metal layer containing a substrate electrode to be formed; PA1 b) depositing over the entire surface a patternable material layer which defines the height of the cavity and contains a sacrificial layer to be formed; PA1 c) patterning the patternable material layer and the first metal layer in a single, first photoresist step by etching for simultaneously forming the substrate electrode, first interconnection tracks connected therewith, and the sacrificial layer, which is practically congruent with the base electrode and the first interconnection tracks, thereby partially exposing the glass substrate; PA1 d) depositing a first insulating layer containing the diaphragm over the entire surface, so that said first insulating layer firmly adheres to the areas of the glass substrate exposed in step c), even in an edge region next to the sacrificial layer; PA1 e) forming, in a photoresist layer deposited over the entire surface, a photoresist mask whose opening is congruent with a top electrode to be formed, which will extend onto the edge region of the first insulating layer next to the sacrificial layer, and with second interconnection tracks connected with the top electrode; PA1 f) depositing a second metal layer containing the top electrode over the entire surface of the photoresist mask; PA1 g) removing the photoresist mask with the overlying portions of the second metal layer by a lift-off step; PA1 h) etching away the portions of the first insulating layer not covered by the top electrode and by the second interconnection tracks; PA1 i) removing the sacrificial layer by lateral etching, starting from its portions lying on the first interconnection tracks, and PA1 k) hermetically sealing the cavity by depositing a second insulating layer over the entire surface in a vacuum. PA1 a) depositing a first metal layer over the entire surface of a glass substrate serving as the base element, said first metal layer containing a substrate electrode to be formed; PA1 b) depositing over the surface entire a patternable material layer which defines the height of the cavity and contains a sacrificial layer to be formed; PA1 c') patterning the patternable material layer and the first metal layer in a single, first photoresist step by etching for simultaneously forming the substrate electrode, corner pads connected therewith, and the sacrificial layer, which is practically congruent with the substrate electrode and the corner pads, thereby partially exposing the glass substrate; PA1 d') depositing a first insulating layer containing the diaphragm over the entire surface, so that said insulating layer firmly adheres to the areas of the glass substrate exposed in step c'), even in four lateral regions next to the sacrificial layer, and etching openings into the first insulating layer for the subsequent supply of an etchant to the sacrificial layer, said openings lying over at least a part of the four corner pads of one absolute pressure sensor; PA1 e') forming, in a photoresist layer deposited over the entire surface, a photoresist mask whose opening is congruent with a top electrode to be formed and with interconnection tracks connected therewith and is centered with the substrate electrode without the corner pads; PA1 f) depositing a second metal layer containing the top electrode over the entire surface of the photoresist mask; PA1 g) removing the photoresist mask with the overlying portions of the second metal layer by a lift-off step; PA1 i') removing the sacrificial layer by vertical etching and lateral etching through the openings, and PA1 k') hermetically sealing the openings, and thus the cavity, by depositing a second insulating layer in a vacuum. PA1 a) depositing a first metal layer over the entire surface of a glass substrate serving as the base element, said first metal layer containing a substrate electrode to be formed; PA1 b") depositing over the entire surface a first patternable material layer containing a first partial layer of a sacrificial layer to be formed, said first partial layer defining a first part of the height of the cavity; PA1 c") patterning the patternable material layer and the first metal layer in a single, first photoresist step by etching for simultaneously forming the substrate electrode, first interconnection tracks connected therewith, and the first partial layer, which is practically congruent with the substrate electrode and the first interconnection tracks; PA1 c"') depositing over the entire surface a second patternable material layer which defines the remainder of the height of the cavity and contains a second partial layer of the sacrificial layer to be formed and two diametrically opposed corner extensions, and patterning said second patternable material layer in a second photoresist step so that it completely covers the first partial layer; PA1 d") depositing a first insulating layer containing the diaphragm over the entire surface, so that said first insulating layer firmly adheres to the areas of the glass substrate still exposed after step c"'), and etching openings into the first insulating layer above the corner extension for the subsequent supply of an etchant to the sacrificial layer; PA1 e") forming, in a photoresist layer deposited over the entire surface, a photoresist mask whose opening is congruent with a top electrode to be formed and with second interconnection tracks connected therewith and is centered with the substrate electrode; PA1 f) depositing a second metal layer containing the top electrode over the entire surface of the photoresist mask; PA1 g) removing the photoresist mask with the overlying portions of the second metal layer by a lift-off step; PA1 i') removing the sacrificial layer by vertical etching and lateral etching through the openings, and PA1 k') hermetically sealing the openings, and thus the cavity, by depositing a second insulating layer in a vacuum. PA1 a') depositing a first metal layer over the entire surface of a glass substrate serving as the base element, said first metal layer containing a substrate electrode to be formed, and subsequently patterning said first metal layer in a first photoresist step for forming the substrate electrode and first interconnection tracks connected therewith; PA1 b"') depositing over the entire surface a first patternable material layer which defines a first part of the height of the cavity and contains a first partial layer of a sacrificial layer to be formed, and patterning said first patternable material layer in a second photoresist step so that it completely covers the substrate electrode; PA1 c"") depositing over the entire surface a second patternable material layer which defines the remainder of the height of the cavity and contains a second partial layer of the sacrificial layer to be formed and two diametrically opposed corner extensions, and patterning said second patternable material layer in a third photoresist step so that it completely covers the first partial layer; PA1 d") depositing over the entire surface a first insulating layer containing the diaphragm, so that said first insulating layer firmly adheres to the areas of the glass substrate still exposed after step c""), and etching openings into the first insulating layer above the corner extensions for the subsequent supply of an etchant to the sacrificial layer; PA1 e") forming, in a photoresist layer deposited over the entire surface, a photoresist mask whose opening is congruent with a top electrode to be formed and with second interconnection tracks connected therewith and is centered with the substrate electrode; PA1 f) depositing a second metal layer containing the top electrode over the entire surface of the photoresist mask; PA1 g) removing the photoresist mask with the overlying portions of the second metal layer by a lift-off step; PA1 i') removing the sacrificial layer by vertical etching and lateral etching through the openings, and PA1 k') hermetically sealing the openings, and thus the cavity, by depositing a second insulating layer in a vacuum.